Kronii is a cuckquean, Mumei is a voyeurist... They'll never have sex, they'll just have an increasingly convoluted game of watching other girls fuck as proxies

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kronii calls you late at night
seductively invites you to her apartment saying that mumei is with her too
you expect a threesome you bring a pack of condoms
they bring their random non hololive friend for you to fuck just so they can watch in the corner and masturbate

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afterwards Kronii begins acting like she's your gf
you're confused because you assumed she was dating Mumei and wasn't interested in fucking you
She (and Mumei, who Kronii has been cuddling the entire time) burst out into laughter, and tell you how silly you are
You agree to date her, but 3 months down the line you're not even in the room for the sex anymore, because Kaela is standing in for you, and the girl they had you fuck has been replaced three times over